LINE (http://line.naver.jp/en/) is another cross platform messenger just like Whatsapp. It also have voice call over data network like Viber. And it's free (not like Whatsapp, it's free for first year and then charges US$0.99/year). So in my area my friends usually have both Whatsapp and LINE and that make the switch between them very easily (not like ebuddy XMS that almost none of my friends are in).

Lately LINE claim that they will provide PC/browser version really soon. I think this is good as it can be accessed at least from browser for the devices (e.g. N9) that are not supported by the nice company (e.g. Whatsapp company) (and maybe it can be integrated into extraplugsin from PC version just same as MSN and ICQ?? my greedy hope )

In my area LINE is quite a strong competitor to Whatsapp and I hope this move from LINE could help "pushing" Whatsapp to do the same thing. Or the PC/browser version of LINE could help providing more ideas of how to port Whatsapp to non-support device (again N9 for example).